You can add them directly to the tank, but make sure to do it in the complete darkness, about one or two hours after the lights go out. If you add them directly to the tank with the lights on, all your inhabitants will be feasting!

p.s. - you are lucky, most of the time a mandarin and scooter wouldn't survive together in a small aquarium; there would be too much competition for food. My Green Mandarin only eats pods. To have both a scooter and a mandarin that eat mysis, that's awesome!